Scanning antenna with power modulation



Oct. 20, 1964 LA VERGNE E. WILLIAMS SCANNING ANTENNA WITH POWERMODULATION Filed April 10, 1961 INVENTOR. L AVEIZGNE E..WILL.\AMS

ATTO 2 N EYS United States Patent "ice 3,153,787 SCANNING ANTENNA WITHPOWER MODULATION La Vergne E. Williams, Indialantic, Fla, assignor toRadiation, Inc., Melbourne, Fla., a corporation of Florida Filed Apr.10, 1961, Ser. No. 102,061 '8 (Ilaims. (Cl. 343-100) The presentinvention relates generally to antenna systems and more particularly topower modulating tracking antenna systems with conical scan.

Power modulation is a method whereby the gain of an antenna is varied asa function of its vertical scan so its gain is considerably reduced whenthe beam is pointed at low angles to the horizon. In angular trackingapplications, this is important because transmission and reception oflow angle signals is degraded due to ground reflections. Reflectionsresult in inaccurate target location indications because the groundreflections do not come from the true target direction.

Prior power modulation systems have usually employed variableattenuators synchronized with the antenna conical scan to introducelarge amounts of signal attenuation when the antenna beam is pointed atlow angles to the horizon and relatively small amounts of attenuationwhen the antenna is at higher vertical angles. These prior systems haveproven undesirable because they are expensive or they utilizecomplicated moving parts requiring maintenance, in addition to asynchronized mechanical following system. Also, the prior devices haveproven unfavorable because they introduce signal attenuation (insertionloss) when none is desired thereby necessitating increased transmitterpower.

Consequently, it is an object of the present invention to provide a newand improved power modulation antenna system which requires no movingparts by itself.

It is a further object of the present invention to provide a powermodulation antenna system that absorbs little or no power when theantenna beam is directed above the ground (maximum vertical angle) inits scan cycle.

Another object is to provide a power modulating antenna system that iseasily adaptable to existing apparatus, is relatively inexpensive, easyto manufacture and service.

Basically, the present invention employs an antenna system having aconical beam pattern. With conical scan the pattern position iscyclically varied in the vertical plane to achieve vertical scan. Anabsorber is disposed adjacent the antenna for absorbing substantialamounts of beam power only when the beam is at low vertical angles,i.e., low angles with respect to the horizon, thereby minimizing thesignal received from reflections to and from the target.

The method of power modulation described herein can be applied to radar(two way) systems or receiving antennas that track an external signalsuch as a beacon, telemetry transmitter, or radio sonde.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIGURE 1 is a schematic diagram of one preferred form of the presentinvention; and

FIGURE 2 is an illustration of the manner in which the nutating cup ofFIGURE 1 is rotated.

FIGURE 1, the schematic diagram of the preferred form of the presentinvention, comprises a stationary dipole antenna 11, connected to aconventional coax line 12, in turn connected to receiver or radarapparatus 13. Dipole antenna 11 is positioned within nutating metal3,153,787 Patented Oct. 20, 1964 reflecting cup 14, having a plasticsupport member 15 located at its open end. Member 15 is a plastic orother suitable material, transparent to electromagnetic energy of thefrequency in which the present system operates. Plastic tube 16, withinwhich coax line 12 is positioned, is connected to support 15 at an oifcenter position. Tube 16 is rotated by motor 17 which drives it throughshaft 18 and gear 19 so cup 14 is an off-center vertically rotating ornutating member. Counter weight 27 is disposed on the surface of cup 14adjacent the cup end closest to the point where tube 16 engages support15 to maintain the mechanical load driven by motor 17 balanced.Stationary absorber 21 made of some suitable material, such as hairflex,is secured to the upper surface of spherical, electromagneticallytransparent, plastic radome 22 which protects the structure enclosedtherein from the weather and other influences. The absorber receivesnone of the antenna energy when cup 14 is in its lower position butreceives a substantial portion of antenna energy when cup 14 is in itsupper position.

This is best shown in FIGURE 2 wherein the upper position of nutatingcup 14 is illustrated by circle 23 and the lower .cup position isillustrated by circle 24. With cup 14 in position 23, absorber 21 is infront of approximately one-half of the reflecting cup and consequently,the effective energy received by or transmitted from antenna 11 isreduced approximately in half. In position 24, no portion of absorber 21is in front of cup 14 and the transmitted or received beam is notaffected by the absorber.

Referring again to FIGURE 1, parabolic reflector 26 is positioned toaffect a pencil beam pattern for primary feed antenna 11 and reversesthe direction and changes the horizontal angle of the signal impingingthereon. Radome 22 is positioned approximately at the focal point ofreflector 26 and secured to the reflector by struts 28 and 29. Thevertical movement of cup 14 provides vertical scan for the pencil beamtransmitted and received by dipole 11 by controlling the direction whichmost of the energy is directed towards or received from dish 26. Withthe cup in its lowest position 24 (FIGURE 2), the secondary pattern ofthe antenna, i.e. that part directed toward cup 14, is directed abovethe axis of the antenna and the ab sorber does not absorb signal energy.When cup 14 is rotated to its upper position 23, the secondary patternis directed below the axis of the antenna and a substantial portion ofthe signal is absorbed by absorber 21. Since energy is either obtainedfrom or directed towards low angles with respect to the horizon, theantenna gain is a minimum for low horizontal angles. Thereby, groundreflections in the transmitted and received signals are sub stantiallyreduced. In this manner, power modulation is obtained with relativelysimple structure requiring no additional moving parts and with a minimumof cost.

Of course, it is to be understood that radar receiver and transmitter 13is positioned at a remote location as is motor 17 and any suitabledriving apparatus associated therewith. The invention can be applied toother conical scan tracking antenna systems such as those employing anutation horn. The disclosed illustration being for schematic purposesonly.

While I have described and illustrated one specific embodiment of myinvention, it will be clear that variations of the details ofconstruction which are specifically illustrated and described may beresorted to without departing from the true spirit and scope of theinvention as defined in the appended claims.

I claim:

1. An antenna system comprising antenna means having a scanned pencilbeam pattern of R.F. electromagnetic energy, means for cyclicallyvarying the vertical position of said pattern and means for absorbingsubstancomprises a n'utating primary feed.

3. A power modulation antenna system comprising an antenna feed systemfor transmitting and receiving a beam of RF. electromagnetic energyhaving a secondary pattern, means for reflecting the beam emanating fromsaid antenna feed system, means for cyclically varying the verticalposition between said antenna feed system and said reflector, means forabsorbing substantial energy from said beam only when the secondarypattern is directed at low angles relative to the horizon.

4. A power modulation antenna system comprising an antenna feed for abeam of R.F. electromagnetic energy, a reflector for said beam, meansfor cyclically varying the vertical position of said beam, and means forabsorbing substantial amounts of energy from said beam only A} when saidcup is in an upper position, and a parabolic reflector cooperating withsaid cup for reflecting said energy impinging thereon. I

6. The system of claim 5 further including means for rotating said cupby the reflector axis, said cup being rotated about a point removed fromits longitudinal axis, said antenna being located at said point, saidabsorber being positioned completely above said cup when said cup is inthe lowest position. i

7. A power modulation antenna system comprising means having a secondaryscanned pencil beam pattern of RP. electromagnetic energy, meansforZyclical ly varying the vertical position of said pattern, and meansfor when said beam is at low angles with respect to the absorbingsubstantial amounts of power from said beam only when it is in theminimum elevation portion of its scan cycle.

,8. The system of claim 4 wherein said means for ab sorbing ispositioned remotely from said reflector and proximately to said feed.

References Cited in the file of this patent UNITED STATES PATENTS2,407,250 Busignies t Sept. 10, 1946 2,419,556 Feldman Apr. 29, 19472,597,313 Gardner May 20, 1947

1. AN ANTENNA SYSTEM COMPRISING ANTENNA MEANS HAVING A SCANNED PENCILBEAM PATTERN OF R.F. ELECTROMAGNETIC ENERGY, MEANS FOR CYLICALLY VARYINGTHE VERTICAL POSITION OF SAID PATTERN AND MEANS FOR ABSORBINGSUBSTANTIAL AMOUNTS OF POWER FROM SAID PATTERN ONLY WHEN SAID PATTERN ISDIRECTED AT LOW ANGLES WITH RESPECT TO THE HORIZON.